(1) Field of the Invention
This invention relates to a process and apparatus for the continuous belt casting of metal strips and, particularly, to the twin-belt casting of metal strips from a variety of molten metals having different cooling requirements and characteristics.
(2) Description of the Related Art
Twin-belt casting of metal strips typically involves the use of a pair of endless belts, usually made of flexible, resilient steel bands or the like, which are driven over suitable rollers and other path defining means, so that they travel together along opposite sides of an elongated narrow space, typically downward-sloping or horizontal, which forms a casting cavity. Molten metal is introduced between the belts in the vicinity of the upstream entry end of the casting cavity and the metal is discharged as a solidified strip or slab from the downstream exit end of the cavity.
An example of a twin-belt casting system can be found in Rochester et al. U.S. Pat. No. 3,163,896, issued Jan. 5, 1965. That patent describes a casting machine in which each belt is circulated, in turn, around a tension roll, a guide roll, at least a pair of sizing rolls and a power roll. The belts are maintained in position to form a casting cavity by the guide rolls and the sizing rolls, such that the cavity after the last sizing roll diverges before feeding onto the power rolls. The sizing rolls, in combination the guide rolls, press against the opposite sides of the belts throughout the cooling and solidification region, and serve to maintain (adjustably, if desired) the selected, predetermined distance between the belts, depending on the thickness desired in the resulting cast strip.
In Hazelett et al. U.S. Pat. No. 3,167,830, issued Feb. 2, 1965, a twin-belt casting apparatus is described in which the upper and lower belt assemblies can be moved with respect to each other so as to affect the cavity length/position. This is used to permit flexibility in the type of operation, e.g. pool vs. direct nozzle feed, and thickness. The flexibility does not affect the cavity length when measured as the total length in which the belt actually confines the slab.
Wood et al. U.S. Pat. No. 4,367,783, issued Jan. 11, 1983, describes a further twin-belt casting system in which load cells are used to measure the pressure applied to a shrinking metal slab and are the results are then used to apply a corrective taper to the cavity. This adjustment to the taper does not affect the length of the cavity.
A still further design is described in Braun et al. WO 97/18049 published May 22, 1997 . This document describes a block caster which can be adapted to have a belt-type liner, and hence behave as a belt caster backed up by a series of connected blocks. The taper of the cavity can be adjusted to meet various metallurgical needs, but there is no description of a system for varying the contact length with the cast strip.
Different alloys, e.g. foil alloys versus can-end or automotive alloys, have remarkably different heat flux requirements, i.e. they require very different heat extraction rates to ensure that a good quality cast slab is obtained. As a result, a caster designed to cast foil alloys, requiring a relatively low heat extraction, will have a relatively long cavity. If the same caster is used with a high heat flux suitable for can-end or similar alloys, the amount of slab cooling that occurs along the cavity is too high and the exit temperature of the slab is too low for subsequent processing (e.g. rolling). If the overall convergence of the cavity is lessened to compensate, the surface quality of the slab deteriorates. Thus, there remains a need for a twin-belt caster that, for a wide range of aluminum alloys, can operate at essentially constant throughput yet ensure that the cast slab exiting the caster has a temperature lying within a predetermined temperature range suitable for further rolling to produce a desired sheet product.